Cant paste flipped pose properly
You've called your bones Left_Shoulder and Right_Shoulder (prefixes), and so on for the other bones, instead of Shoulder_L and Shoulder_R (suffixes). Actually I was a bit surprised that you were able to copy flip with this kind of terminology. That said, it looks like in the Dopesheet, with CtrlC and ShiftCtrlV you still need to use suffixes. I guess it's something they have forgotten to fix: Either you can use prefixes everywhere or you can't, or I miss something...